The spill, which appeared to be black oil, was on the shoreline at Matapihi, south of the rail bridge, and covered an area of shoreline of less than 50m in length.

A slight sheen was also visible on the water.

"Our focus now shifts to an investigation phase and we are looking at where the oil may have come from," Mellor said on Monday morning.

"The most important thing for us at this stage though is if anyone notices any oil at any other place in the harbour, to let us know as soon as possible by ringing the pollution hotline on 0800 884 883 so we can get staff out to the site."

On Sunday evening Mellor said staff were manually picking up as much of the oil as they could and getting it into bags, to stop the oil going back into the harbour.

Mellor also said on Sunday that it was too early to tell what caused the spill or where the oil came from.

"We're taking notes, taking photographs, and cleaning up the oil," he said.

Oil in the water near the Matapihi rail bridge on Sunday. Photo / George Novak